The study examined how student control over metacognitive prompts in a multimedia environment affects students' ability to solve mathematical problems in immediate comprehension tasks using a multimedia program and a delayed-transfer test. It also examined the effect on metacognitive discourse, mental effort, and engagement with multimedia-based tasks. Participants were 90 8th-grade students, randomly assigned to three groups working in pairs: (a) "solicited prompts" group, with free access to multimedia-based metacognitive prompts; (b) "unsolicited prompts" group, consistently and regularly exposed to on-screen metacognitive prompts; and (c) a control group with no multimedia metacognitive prompts. Mixed method analysis showed that of the three groups the unsolicited prompts group had the highest effects in the immediate comprehension tasks (d = 1.23; 1.92 respectively for the solicited prompts group and the control group) and the delayed-transfer problem solving test (d = 0.55; 0.93, respectively). The level of this group's metacognitive discourse was higher, particularly in the planning phase. Group members displayed less cognitive load on the complex tasks and were more engaged in the multimedia activities. In contrast, the solicited prompts group felt they were under a heavier cognitive load than the other two groups. Finding implications and future directions for study are discussed.